In the construction of any battery, six components are universally present: two electrodes (positive and negative), electrolyte, housing, an ion permeable separator between positive and negative electrodes, and the current collectors. The chemical reactions that occur in the electrodes are mediated by ion flow therebetween. Since there is a net change in electric charge, the excess electrons must be transferred for one electrode to the other along a path that generates an electric current.
The current collector functions to gather the electrons at one electrode and discharge them at the other. The efficiency with which electron uptake and discharge takes place affects battery capacity, charge density, and conductivity Thus, current collectors are generally made of sheets of highly conductive materials such an metals. Typical metallic collectors are composed of copper, aluminum, gold, silver, tin, or nickel. In battery construction there are several potential problems relating to the current collectors.
If the surfaces between electrode and collector are uneven, contact will be impaired and there will be surface gaps which interrupt electron flow. Similarly, if contact is maintained by a press fit, the pressure applied must be uniform. Also, there is a tendency for laminated-type batteries, to come apart at the interface of electrode and current collector. Some of these problem are intensified in the field of secondary batteries, since the expansion/contraction phenomenon during charge and discharge cycles causes successive bending of the materials back and forth. The differential coefficients of expansion/contraction of the materials in the battery components tends to pull the layers apart.
In the construction of many batteries, including secondary lithium batteries, various strategies have been developed to maintain contact between current collector and electrode. Pressure laminants contained in rigid battery housings are disclosed in U.S. Pat. No. 5,478,668. U.S. Pat. No. 4,710,439 disclosed a swagelock construction for test cells, in which the components are maintained in contact by mechanical pressure. Another important construction strategy, described representatively in U.S. Pat. No. 4,550,064, is to tightly wind the layers of battery components in a spiral, thus obtaining strong adhesive pressures partially by exerting sheer during the winding.
Adhesion of plastic polymer based electrodes to current collectors in plastic lithium ion secondary batteries is a particularly difficult problem because plastic and metal interfaces do not inherently provide appreciable natural molecular adhesion interactions. Several approaches have been devised to overcome this problem. U.S. Pat. No. 5,591,544 discloses a primer material, preferably carbonaceous, which is interposed between the electrode and an aluminum current collector. The primer improves adhesion presumably because adhesion of both the metal and the plastic to the primer is better than between metal and electrode. Another advantage is that the primer retards oxidation of the aluminum which interferes with conductivity. U.S. Pat. No. 5,589,297 also applies a basement layer to increase adhesion of the current collector.
A different approach is disclosed in U.S. Pat. No. 5,587,253 in which a perforated metal electrode is embedded within the electrode. This has two advantages, namely, the crosslinked polymer communicating covalently with the body of polymer electrode on each side of the collector forms a strong unitary structure, and secondly, the distance of travel for electrons is half the distance for an electrode of the same thickness in which the collector is applied to only one side. The disadvantage is loss of current collector surface area because of the perforations. In an alternate embodiment of this concept, U.S. Pat. No. 5,554,459 utilizes an embedded metallic grid rather than perforated metal. In a third approach, U.S. Pat No. 5,584,893 discloses that application of electrode directly to the current electrode as a slurry improves adhesion.
From the foregoing, it is apparent that a prominent challenge in the battery art is to obtain intimate adhesion of metallic current collector to electrode, particularly plastic-based electrodes, while at the same time maintaining or even increasing the surface area of contact between collector and electrode. The improvement in adhesion should be such that the cell is highly resistant to debonding between electrode and current collector during cycling of secondary batteries.